ADCMP605 Analog Devices, ADCMP605 Datasheet - Page 11

• Manufacturer Part Number: ADCMP605
• Description: (ADCMP604 / ADCMP605) Single-Supply LVDS Comparators
• Manufacturer: Analog Devices

The ADCMP604 and ADCMP605 dispersion is typically < 1.6 ns
as the overdrive varies from 10 mV to 125 mV. This specification
applies to both positive and negative signals because each the
ADCMP604 and ADCMP605 have substantially equal delays for
positive-going and negative-going inputs and very low output
skews.
COMPARATOR HYSTERESIS
The addition of hysteresis to a comparator is often desirable in a
noisy environment, or when the differential input amplitudes
are relatively small or slow moving. The transfer function for a
comparator with Hysteresis is shown in Figure 19. As the input
voltage approaches the threshold (0.0 V, in this example) from
below the threshold region in a positive direction, the
comparator switches from low to high when the input crosses
+V
comparator remains in the high state until the threshold, −V
is crossed from below the threshold region in a negative direction.
In this manner, noise or feedback output signals centered on
0.0 V input cannot cause the comparator to switch states unless it
exceeds the region bounded by ±V
H
/2. The new switching threshold becomes −V
INPUT VOLTAGE
INPUT VOLTAGE
Q/Q OUTPUT
Q/Q OUTPUT
Figure 17. Propagation Delay—Overdrive Dispersion
Figure 18. Propagation Delay—Slew Rate Dispersion
10V/ns
1V/ns
H
/2.
500mV OVERDRIVE
10mV OVERDRIVE
DISPERSION
DISPERSION
V
V
N
N
± V
H
± V
/2. The
OS
OS
H
Rev. 0 | Page 11 of 16
/2,
The customary technique for introducing hysteresis into a
comparator uses positive feedback from the output back to the
input. One limitation of this approach is that the amount of
hysteresis varies with the output logic levels, resulting in
hysteresis that is not symmetric about the threshold. The
external feedback network can also introduce significant
parasitics that reduce high speed performance and induce
oscillation in some cases.
The ADCMP605 comparator offers a programmable hysteresis
feature that significantly improves accuracy and stability.
Connecting an external pull-down resistor or a current source
from the LE/HYS pin to GND, varies the amount of hysteresis
in a predictable and stable manner. Leaving the LE/HYS pin
disconnected or driving it high removes hysteresis. The
maximum hysteresis that can be applied using this pin is
approximately 160 mV. Figure 20 illustrates the amount of
hysteresis applied as a function of external resistor value.
Figure 11 illustrates hysteresis as a function of current.
The hysteresis control pin appears as a 1.25 V bias voltage seen
through a series resistance of 7 kΩ ± 20% throughout the
hysteresis control range. The advantages of applying hysteresis
in this manner are improved accuracy, improved stability,
reduced component count, and maximum versatility. An
external bypass capacitor is not recommended on the HYS pin
because it would likely degrade the jitter performance of the
device and impair the latch function. As described in the
Using/Disabling the Latch Feature section, hysteresis control
need not compromise the latch function.
Figure 19. Comparator Hysteresis Transfer Function
–V
2
H
OUTPUT
V
OL
ADCMP604/ADCMP605
0
V
OH
+V
2
H
INPUT